I may say by directing, to-day, our thoughts to one kind of power. You see what I mean by the term matter—any of these things that I can lay hold of with the hand, or in a bag (for I may take hold of the air by enclosing it in a bag)—they are all portions of matter with which we have to deal at present, generally or particularly, as I may require to illustrate my subject. Here is the sort of matter which we call water,—it is there ice [pointing to a block of ice upon the table], there water [pointing to the water boiling in a flask], here vapour—you see it issuing out from the top [of the flask]. Do not suppose that that ice and that water are two entirely different things, or that the steam rising in bubbles and ascending in vapour there is absolutely different from the fluid water. It may be different in some particulars, having reference to the amounts of power which it contains; but it is the same, nevertheless, as the great ocean of water around our globe, and I employ it here for the sake of illustration, because if we look into it we shall find that it supplies us with examples of all the powers to which I shall have to refer. For in-
